With depletion of global liquid petroleum reserves, natural gas, containing primarily methane, is expected to be one of the main resources for the production of liquid fuels. However, direct dehydrogenation of light alkanes like methane and ethane to more valuable olefins remains challenging.
For ethane to olefin production, commercial processes include steam cracking and catalytic dehydrogenation, and recently there has also been renewed interest in oxidative dehydrogenation.
Oxidative dehydrogenation offers direct conversion from alkanes into valuable chemicals. By adding oxygen discretely through either porous or dense oxygen permeable membranes, the alkane to oxygen ratio can be kept high, promoting high C2+ selectivity.
A somewhat less investigated route for alkane conversion to fuels is through non-oxidative reactions. Here, using methane as an example, a coupling/dimerization/pyrolysis (hereafter denoted coupling) reaction takes place on the methane side of a reactor with hydrogen permeating through a membrane in the form of protons onto the oxygen side, where it reacts with oxygen to form water.
Oxygen is not present in the methane coupling compartment, avoiding the oxidation of methane. A high C2+ selectivity may thus be expected. This is a highly efficient way to make olefins from alkanes compared to existing technologies.
It has been shown theoretically that removal of hydrogen during coupling promotes homogeneous reaction pathways and shifts the equilibrium towards the product side. A hydrogen selective membrane in the process stream should therefore increase the yield considerably. The removal of hydrogen can be achieved using hydrogen permeable membranes.
Several such membranes exist. Catalytic dehydrogenation of ethane in a hydrogen membrane reactor has been investigated using a microporous silica membrane and a 5.0 wt. % Cr2O3/γ-Al2O3 catalyst prepared by incipient wetness impregnation of a γ-Al2O3 support.
A Pd—Ag composite membrane supported on porous stainless steel prepared by electroless plating has been used in a catalytic membrane reactor utilizing a Ru—Mo/HZSM-5 catalyst.
Using the ceramic mixed proton-electron conductor SrCe0.95Yb0.05O3-δ a membrane configuration and also a co-generative fuel cell has been developed towards methane coupling.
There are problems with all these solutions however. Microporous membranes suffer from being fragile and difficult to make. Their hydrogen selectivity is also poor.
Pd—Ag membranes are inherently very expensive and whilst complex membranes have been formed in an attempt to minimise expensive metal content, there remains a desire to have a much simpler membrane. The catalytic activity of these metals towards formation of coke is also a considerable problem if these materials are used in a catalytic membrane reactor.
Ceramic oxides offer a more attractive option therefore. However, even initiatives using ceramic proton conducting materials have serious limitations. The prior art ceramic oxides are based on Ba- and Sr-based perovskites. These compounds are basic and are prone to react with CO2 and H2S/SO2/SO3 at moderate temperature and H2O at low temperatures to form alkaline earth carbonates, sulphates and hydroxides, respectively. Consequently, a decrease in conductivity is observed.
These reactions are prohibitive if using any carbon-containing feed gas as the impurities in the gas react with the membrane. Moreover, the reaction with carbon dioxide precludes the use of air in a reactor meaning expensive inert gases have to be used. Moreover, the electrical and mechanical properties of these materials become poor due to the formation of carbonates and hydroxides.
The use of mixed metal tungstates offers an ideal solution to this problem. These materials are stable in the presence of carbon dioxide and acidic gases in general making them usable in the presence of air. This also means the membranes can be used in the presence of hydrocarbon feed gases.
Mixed metal tungstates are not themselves new. In Solid State Ionics, 143 (2001), 117-123, the authors investigate the proton conducting properties of lanthanum tungstates. The present inventors have realised that these proton conducting materials, as opposed to the numerous other proton conducting materials known, offer the most attractive properties for use in dehydrogenation reactions, in particular of alkanes to alkenes (olefins/aromatics).
Ceramic materials selected from a group of rare-earth tungstates, often in literature denoted with formulas Ln6-xWO12-δ, where Ln refers to lanthanides or rare earth elements, have been known to have proton conducting properties for some time (Yoshimura et al. Materials Research Bulletin 10 (1975) (9) 983) but have received increasing interest after being examined by Shimura et al. and later Haugsrud et al. (Shimura et al. Solid State Ionics 143 (2001) 117, Haugsrud et al. J. Phys. Chem. Solids 69 (2008) (7) 1758). Rare-earth tungstates exhibit highest proton conductivity when nominally undoped, a characteristic that sets them apart from the more well characterized proton conductors of pervoskite systems such as acceptor doped SrCeO3 or BaCeO3. Rare-earth tungstates of formula Ln6-xWO12-δ are known to exhibit mixed proton and electron conductivity (n-type conductivity) in reducing atmospheres and mixed proton and electron hole conductivity (p-type conductivity) under oxidizing conditions (Escolastico et al. Chem. Mat 21 (2009) (14) 3079 and Haugsrud et al. J. Phys. Chem. Solids 69 (2008) (7) 1758). An increase in conductivity with increasing reducing or oxidizing conditions indicates dominating n- and p-type conduction respectively.
Hence, rare-earth tungstates are today considered by many to be a promising candidate for hydrogen separation membranes where they are solely used under reducing conditions and for electrode materials in proton conducting solid oxide fuel cells (see, for example, Haugsrud et al. J. Phys. Chem. Solids 69 (2008) (7) 1758, Escolastico et al. Chem. Mat. 21 (2009) (14) 3079; Solis et al. Journal of Physical Chemistry C 115 (2011) (22) 11124; Escolastico et al. International Journal of Hydrogen Energy 36(18) (2011) 11946-11954; Solis et al Journal of Materials Chemistry 22 (31) (2012) 16051-16059) where the n-type conduction is utilized in a membrane and either the n-type or the p-type partial conduction is utilized in the anode or cathode electrode, respectively.
The use of n-type and p-type conduction in a ceramic membrane comprising rare-earth tungstates has been reported. This is achieved by way of electron conduction (n-type) in one portion of the ceramic membrane and by way of electron hole conduction (p-type) in another portion of the membrane, enabling the transport of hydrogen, dissociated as protons and electrons/electron holes, across the membrane. Consequently, these types of ceramic membranes comprise at least two layers, each comprising a different tungstate material.
Despite the attractive proton conductivity properties of these known rare-earth tungstates, the n-type electronic conductivity, especially at temperatures below 750° C., appears to limit the hydrogen permeation process, especially in La6-xWO12-δ (LWO) which presents one of the highest protonic conductivity of the Ln6-xWO12-δ series. There remains therefore a need to develop new membrane materials which maintain the optimum proton conductivity properties and stability, especially in CO2-rich environments, of the tungstates but which also overcome the problems associated with the poor electronic conductivity of these membranes.
The present inventors suggest the use of a combination of two mixed metal oxides in the manufacture of a proton conducting ceramic membrane. One of those mixed metal oxides is preferably based on a lanthanum tungstate.
The successful synthesis of doped Ln6W1.1O12-δ compounds as single phase materials (defective fluorite) has been reported by Escolastico et al PhD Dissertation thesis, 2013; J. M. Serra, S. Escolástico, M. Ivanova, W. Meulenberg, J. Seeger, C. Solis, Hydrogen permeation through La5.5WO12 membranes presented at 10th CMCee—International Symposium on ceramic materials and components for energy and environmental applications, 20-23 May 2012, Dresden (Germany). These compositions comprised (Ln1-xAx)6W1.1O12-δ compounds, where A=lanthanides and/or alkali-earths, and, especially, (Ln1-xAx)6(W1-yBy)1.1O12-δ where x=0.1, 0.5, 1 and B=Mo, Re, Cr, Nb, U, among others. The preparation was carried out principally by a sol-gel and pyrolysis method. Complete electrochemical conductivity measurements have been carried out for most of them, making special emphasis on the promotion of n-type conductivity in reducing gas environments and p-type conductivity in oxidizing gas environments.
Some examples of the tested samples:
Nd5LaW1.1O12-δ, Nd5CeW1.1O12-δ, Nd5PrW1.1O12-δ, Nd5EuW1.1O12-δ, Nd5TbW1.1O12-δ, Nd5SmW1.1O12-δ, and, Nd6WMo0.1O12-δ, Nd6W0.6Mo0.5O12-δ, Nd6WRe0.1O12-δ, Nd6W0.6Re0.5O12-δ, Nd6WU0.1O12-δ, Nd6W0.6U0.5O12-δ, Nd6WCr0.1O12-δ, Nd6W0.6Cr0.5O12-δ, Nd6WNb0.1O12-δ and Nd6W0.6Nb0.5O12-δ. Similar doped compositions for La5.4WO12-δ doped with Nd, Ce, Tb, Y, Mo, Re and Ir are available in Seeger et al. Inorganic Chemistry 52 (2013) 10375-10386, Amsif et at. Chemistry of materials 24 (20) (2012) 3868-3877; Escolatico et al Solid State Ionics 216 (2012 31-35); and Zayas-Rey et al. Chemistry of Materials 25 (2013) 448-456.
Two general approaches are known in the art to be of value for increasing electronic conductivity in tungstates, (a) selective doping of the defective fluorite structure; and (b) “physical” mixing with a good electronic conducting phase. Approach (b) has been applied broadly in oxygen transport membranes (Angew. Chem. Int. Ed. 2011, 50, 759-763) and in hydrogen transport membranes (J. Power Sources 159 (2006) 1291-1295), the latter case involving blending the proton conductor with metals.
The present inventors have surprisingly found that blending a rare-earth tungstate (LWO) with a mixed metal oxide in a conducting layer makes it possible to substantially increase the “overall” ambipolar conductivity of the material. By having a balanced proportion of the LWO and mixed metal oxide phases, the hydrogen permeation flux of the LWO can be increased up to 5-fold when compared to single material membranes. Best results are obtained when the two phases are mixed as powders with grain sizes of less than a few μm. This maximises the electronic conductance effect, as well as the promotion of grain boundary effects.
Also of interest is separation of hydrogen from steam reformed natural gas. The membranes dissolve hydrogen gas as protons and electrons. The production of hydrogen from natural gas by steam reforming is a well-known art. The reaction is favoured by high temperature. By extracting hydrogen by means of a hydrogen membrane, the steam reforming reaction is shifted to the right and more hydrogen is produced.
The possibility of using a hydrogen permeable membrane in steam reforming has been investigated earlier as described in US 2004/0241071. One membrane composition utilized is a single phase of the mixed metal oxide used in the present invention. The material suffers from low proton conductivity at operating temperatures, however, and has not been proven commercially.